Accessing a multi-level memory cell

ABSTRACT

Methods, systems, and devices for accessing a multi-level memory cell are described. The memory device may perform a read operation that includes pre-read portion and a read portion to access the multi-level memory cell. During the pre-read portion, the memory device may apply a plurality of voltages to a plurality of memory cells to identify a likely distribution of memory cells storing a first logic state. During the read portion, the memory device may apply a first read voltage to a memory cell based on performing the pre-read portion. The memory device may apply a second read voltage to the memory cell during the read portion that is based on the first read voltage. The memory device may determine the logic state stored by the memory cell based on applying the first read voltage and the second read voltage.

CROSS REFERENCE

The present Application for Patent is a continuation of U.S. patentapplication Ser. No. 16/926,556 by Sarpatwari et al., entitled“ACCESSING A MULTI-LEVEL MEMORY CELL,” filed Jul. 10, 2020, assigned tothe assignee hereof, and is expressly incorporated by reference in itsentirety herein.

BACKGROUND

The following relates generally to one or more systems for memory andmore specifically to accessing a multi-level memory cell.

Memory devices are widely used to store information in variouselectronic devices such as computers, wireless communication devices,cameras, digital displays, and the like. Information is stored byprogramming memory cells within a memory device to various states. Forexample, binary memory cells may be programmed to one of two supportedstates, often denoted by a logic 1 or a logic 0. In some examples, asingle memory cell may support more than two states, any one of whichmay be stored. To access the stored information, a component of thedevice may read, or sense, at least one stored state in the memorydevice. To store information, a component of the device may write, orprogram, the state in the memory device.

Various types of memory devices exist, including magnetic hard disks,random access memory (RAM), read-only memory (ROM), dynamic RAM (DRAM),synchronous dynamic RAM (SDRAM), ferroelectric RAM (FeRAM), magnetic RAM(MRAM), resistive RAM (RRAM), flash memory, phase change memory (PCM),and others. Memory devices may be volatile or non-volatile. Non-volatilememory, e.g., FeRAM, may maintain their stored logic state for extendedperiods of time even in the absence of an external power source.Volatile memory devices, e.g., DRAM, may lose their stored state whendisconnected from an external power source. FeRAM may be able to achievedensities similar to volatile memory but may have non-volatileproperties due to the use of a ferroelectric capacitor as a storagedevice.

Improving memory devices, generally, may include increasing memory celldensity, increasing read/write speeds, increasing reliability,increasing data retention, reducing power consumption, or reducingmanufacturing costs, among other metrics. Storing multiple bits ofinformation in a memory cell may be desired to increase a data storagedensity without increasing a physical memory cell density.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example memory device that supports accessing amulti-level memory cell in accordance with examples as disclosed herein.

FIG. 2 illustrates an example of a memory array that supports accessinga multi-level memory cell in accordance with examples as disclosedherein.

FIG. 3A illustrates an example of a diagram showing distributions ofthreshold voltages in a self-selecting memory cell that supportsaccessing a multi-level memory cell in accordance with examples asdisclosed herein.

FIG. 3B illustrates an example of a timing diagram that supportsaccessing a multi-level memory cell in accordance with examples asdisclosed herein.

FIG. 4A illustrates an example of a diagram showing distributions ofthreshold voltages in a self-selecting memory cell that supportsaccessing a multi-level memory cell in accordance with examples asdisclosed herein.

FIG. 4B illustrates an example of a timing diagram that supportsaccessing a multi-level memory cell in accordance with examples asdisclosed herein.

FIG. 5 shows a block diagram of a memory device that supports accessinga multi-level memory cell in accordance with examples as disclosedherein.

FIGS. 6 through 8 show flowcharts illustrating a method or methods thatsupport accessing a multi-level memory cell in accordance with examplesas disclosed herein.

DETAILED DESCRIPTION

A self-selecting memory cell including a chalcogenide material may be anexample of a multi-level cell that is configured to store three or moreunique states. As such, a single multi-level self-selecting memory cellmay be configured to store more than one bit of data. In some cases, aself-selecting memory cell may be selected by applying a bias between aword line and a digit line. The logic state that is stored in aself-selecting memory cell may be based on a polarity of a programmingpulse applied to the self-selecting memory cell and the polarity of aread pulse used to detect the state stored by the self-selecting memorycell. For some multi-level self-selecting memory cells, to program oneor more intermediate memory states to the self-selecting memory cell, aprogramming pulse sequence that includes two pulses may be used. In somecases, one or more pulses have the same or different polarities or thesame or different magnitudes may be used.

Devices, systems, and techniques are described for accessing amulti-level self-selecting memory cell that stores three or more states.A read operation for a multi-level self-selecting memory cell mayinclude two or more portions, including a pre-read portion and a readportion. During the pre-read portion, a plurality of different voltagesmay be applied to a plurality of partitions of a memory device. Thememory device may identify a likely distribution associated with memorycells that store a first logic state. From that distribution, the memorydevice may select a first read voltage to distinguish between storingthe first logic state and a second logic state based on performing thepre-read portion. The memory device may also select a second readvoltage to distinguish between storing the second logic state and athird logic state based on the first read voltage. The memory device maydetermine the logic state based on applying the first read voltage andthe second read voltage.

Accessing the multi-level memory cell by applying a plurality ofvoltages during the pre-read portion and applying a first and secondread voltage during the read portion may improve read margins of a readoperation by selecting read voltages based on current conditions in amemory device. In some examples, these techniques may conserve currentand power by selecting the first read voltage based on the voltagesapplied during the pre-read portion. In such cases, a multi-level readthat stores three or more logic states per memory cell may increase aquantity of bits accessed per memory cell, thereby improving theperformance of the self-selecting memory cell.

Features of the disclosure are initially described in the context ofmemory arrays as described with reference to FIGS. 1-2 . Features of thedisclosure are described in the context diagrams showing distributionsof threshold voltages and timing diagrams as described with reference toFIGS. 3-4 . These and other features of the disclosure are furtherillustrated by and described with reference to an apparatus diagram andflowcharts that relate to accessing a multi-level memory cell asdescribed with reference to FIGS. 5-8 .

FIG. 1 illustrates an example memory device 100 that supports accessinga multi-level memory cell in accordance with examples as disclosedherein. Memory device 100 may also be referred to as an electronicmemory apparatus. The components and features of the memory device 100are shown to illustrate functional interrelationships, may notillustrate their actual physical positions within the memory device 100.The memory device 100 includes a three-dimensional (3D) memory array.The memory array includes memory cells 105 that may be programmable tostore different states. In some examples, each memory cell 105 may beprogrammable to store two states, denoted as a logic 0 and a logic 1. Insome examples, a memory cell 105 may be configured to store more thantwo logic states. A memory cell 105 may, in some examples, include aself-selecting memory cell. Although some elements included in FIG. 1are labeled with a numeric indicator, other corresponding elements arenot labeled, though they are the same or would be understood to besimilar, in an effort to increase visibility and clarity of the depictedfeatures.

The 3D memory array may include two or more two-dimensional (2D) memoryarrays formed on top of one another. This may increase a quantity ofmemory cells that may be placed or created on a single die or substrateas compared with 2D arrays, which in turn may reduce production costs,or increase the performance of the memory device, or both. Based on theexample depicted in FIG. 1 , memory array includes two levels of memorycells 105 and may thus be considered a 3D memory array; however, thequantity of levels is not limited to two. Each level may be aligned orpositioned so that memory cells 105 may be aligned (exactly,overlapping, or approximately) with one another across each level,forming a memory cell stack 145. In some cases, the memory cell stack145 may include multiple self-selecting memory cells laid on top ofanother while sharing an access line for both as explained below. Insome cases, the self-selecting memory cells may be multi-levelself-selecting memory cells configured to store more than one bit ofdata using multi-level storage techniques.

In some examples, each row of memory cells 105 is connected to an accessline 110, and each column of memory cells 105 is connected to a bit line115. Access lines 110 and bit lines 115 may be substantiallyperpendicular to one another and may create an array of memory cells.The two memory cells 105 in a memory cell stack 145 may share a commonconductive line such as a bit line 115. That is, a bit line 115 may bein electronic communication with the bottom electrode of the uppermemory cell 105 and the top electrode of the lower memory cell 105.Other configurations may be possible, for example, a third material mayshare a access line 110 with a lower material. In general, one memorycell 105 may be located at the intersection of two conductive lines suchas a access line 110 and a bit line 115. This intersection may bereferred to as a memory cell's address. A target memory cell 105 may bea memory cell 105 located at the intersection of an energized accessline 110 and bit line 115; that is, access line 110 and bit line 115 maybe energized in order to read or write a memory cell 105 at theirintersection. Other memory cells 105 that are in electroniccommunication with (e.g., connected to) the same access line 110 or bitline 115 may be referred to as untargeted memory cells 105.

As discussed above, electrodes may be coupled to a memory cell 105 andan access line 110 or a bit line 115. The term electrode may refer to anelectrical conductor, and in some cases, may be employed as anelectrical contact to a memory cell 105. An electrode may include atrace, wire, conductive line, conductive material, or the like thatprovides a conductive path between elements or components of memorydevice 100. In some examples, a memory cell 105 may include achalcogenide material positioned between a first electrode and a secondelectrode. One side of the first electrode may be coupled to an accessline 110 and the other side of the first electrode to the chalcogenidematerial. In addition, one side of the second electrode may be coupledto a bit line 115 and the other side of the second electrode to thechalcogenide material. The first electrode and the second electrode maybe the same material (e.g., carbon) or different.

Operations such as reading and writing may be performed on memory cells105 by activating or selecting access line 110 and digit line 115. Insome examples, access lines 110 may also be known as word lines 110, andbit lines 115 may also be known digit lines 115. References to wordlines and bit lines, or their analogues, are interchangeable withoutloss of understanding or operation. Activating or selecting a word line110 or a digit line 115 may include applying a voltage to the respectiveline. Word lines 110 and digit lines 115 may be made of conductivematerials such as metals (e.g., copper (Cu), aluminum (Al), gold (Au),tungsten (W), titanium (Ti)), metal alloys, carbon, conductively-dopedsemiconductors, or other conductive materials, alloys, compounds, or thelike.

Accessing memory cells 105 may be controlled through a row decoder 120and a column decoder 130. For example, a row decoder 120 may receive arow address from the memory controller 140 and activate the appropriateword line 110 based on the received row address. Similarly, a columndecoder 130 may receive a column address from the memory controller 140and activate the appropriate digit line 115. Thus, by activating a wordline 110 and a digit line 115, the memory cell 105 at their intersectionmay be accessed.

Upon accessing, a memory cell 105 may be read, or sensed, by sensecomponent 125 to determine the stored state of the memory cell 105. Forexample, a voltage may be applied to a memory cell 105 (using thecorresponding word line 110 and bit line 115) and the presence of aresulting current may depend on the applied voltage and the thresholdvoltage of the memory cell 105. In some cases, more than one voltage maybe applied. Additionally, if an applied voltage does not result incurrent flow, other voltages may be applied until a current is detectedby sense component 125. By assessing the voltage that resulted incurrent flow, the stored logic state of the memory cell 105 may bedetermined. In some cases, the voltage may be ramped up in magnitudeuntil a current flow is detected. In other cases, determined voltagesmay be applied sequentially until a current is detected. Likewise, acurrent may be applied to a memory cell 105 and the magnitude of thevoltage to create the current may depend on the electrical resistance orthe threshold voltage of the memory cell 105.

Sense component 125 may include various transistors or amplifiers inorder to detect and amplify a difference in the signals, which may bereferred to as latching. The detected logic state of memory cell 105 maythen be output through column decoder 130 as output 135. In some cases,sense component 125 may be part of a column decoder 130 or row decoder120. Or, sense component 125 may be connected to or in electroniccommunication with column decoder 130 or row decoder 120. The sensecomponent may be associated either with column decoder or row decoderwithout losing its functional purposes.

A memory cell 105 may be set or written by similarly activating therelevant word line 110 and digit line 115 and at least three logicvalues may be stored in the memory cell 105. In some cases, more thanthree logic values may be stored in the memory cell 105. Column decoder130 or row decoder 120 may accept data, for example input/output 135, tobe written to the memory cells 105. In the case of a self-selectingmemory cell including a chalcogenide material, a memory cell 105 may bewritten to store data by applying a plurality of pre-read voltagesduring a pre-read portion and then applying a first read voltage and asecond read voltage during a read portion. The first read voltage andsecond read voltage may vary in magnitude and polarity. This process isdiscussed in more detail below with reference to FIGS. 3A, 3B, 4A, and4B.

The memory controller 140 may control the operation (e.g., read, write,re-write, refresh, discharge) of memory cells 105 through the variouscomponents, for example, row decoder 120, column decoder 130, and sensecomponent 125. In some cases, one or more of the row decoder 120, columndecoder 130, and sense component 125 may be co-located with the memorycontroller 140. Memory controller 140 may generate row and columnaddress signals in order to activate the desired word line 110 and digitline 115. Memory controller 140 may also generate and control variousvoltages or currents used during the operation of memory device 100.

The memory controller 140 may be configured to access a multi-levelmemory cell. For example, the memory controller 140 may be configured toexecute a pre-read portion and a read portion to access the multi-levelmemory cell. During the pre-read portion, the memory controller 140 mayapply a plurality of different voltages to a plurality of partitions ofa memory device (e.g., each partition including a quantity of memorycells). The memory controller 140 may identify a distribution associatedwith memory cells that store a first logic state.

Based on the identified distribution, the memory controller 140 mayselect a first read voltage to distinguish between storing the firstlogic state and a second logic state and apply the first read voltage aspart of the read portion. The memory controller 140 may then select asecond read voltage to distinguish between storing the second logicstate and a third logic state, and apply the second read voltage as partof the read portion. The selected second read voltage may be based onthe selected first read voltage. The memory device may determine thelogic state (e.g., first, second, or third logic states) based onapplying the first read voltage and the second read voltage. Accessingthe multi-level memory cell via the pre-read portion and the readportion may conserve current and improve the performance of themulti-level memory cell.

FIG. 2 illustrates an example of a memory array 200 that supportsaccessing a multi-level memory cell in accordance with examples asdisclosed herein. Memory array 200 may be an example of portions ofmemory array described with reference to FIG. 1 . Memory array 200 mayinclude a first array or deck 205 of memory cells that is positionedabove a substrate 204 and second array or deck 210 of memory cells ontop of the first array or deck 205. Memory array 200 may also includeword line 110-a and word line 110-b, and bit line 115-a, which may beexamples of word line 110 and bit line 115, as described with referenceto FIG. 1 . Memory cells of the first deck 205 and the second deck 210each may have one or more self-selecting memory cell. Although someelements included in FIG. 2 are labeled with a numeric indicator, othercorresponding elements are not labeled, though they are the same orwould be understood to be similar, in an effort to increase visibilityand clarity of the depicted features.

Self-selecting memory cells of the first deck 205 may include firstelectrode 215-a, chalcogenide material 220-a, and second electrode225-a. In addition, self-selecting memory cells of the second deck 210may include a first electrode 215-b, chalcogenide material 220-b, andsecond electrode 225-b. The self-selecting memory cells of the firstdeck 205 and second deck 210 may, in some examples, have commonconductive lines such that corresponding self-selecting memory cells ofeach deck 205 and 210 may share bit lines 115 or word lines 110 asdescribed with reference to FIG. 1 . For example, first electrode 215-bof the second deck 210 and the second electrode 225-a of the first deck205 may be coupled to bit line 115-a such that bit line 115-a is sharedby vertically adjacent self-selecting memory cells.

The architecture of memory array 200 may be referred to as a cross-pointarchitecture in which a memory cell is formed at a topologicalcross-point between a word line and a bit line as illustrated in FIG. 2. Such a cross-point architecture may offer relatively high-density datastorage with lower production costs compared to other memoryarchitectures. For example, the cross-point architecture may have memorycells with a reduced area and, resultantly, an increased memory celldensity compared to other architectures. For example, DRAM may use atransistor, which is a three-terminal device, as the selection componentfor each memory cell and may have a larger memory cell area compared tothe cross-point architecture.

In some architectures, a plurality of word lines may be formed onparallel planes or tiers parallel to a substrate. The plurality of wordlines may be configured to include a plurality of holes to allow aplurality of bit lines formed orthogonally to the planes of word linessuch that each of the plurality of bit lines penetrates through avertically aligned set of holes (e.g., the bit lines vertically disposedwith respect to the planes of word lines and the horizontal substrate).Memory cells including storage element (e.g., self-selecting memorycells including a chalcogenide material) may be formed at the crossingsof word lines and bit lines (e.g., spaces between the word lines and thebit line in the vertically aligned set of holes). In a similar fashionas described above with reference to FIG. 1 , the memory cells (e.g.,self-selecting memory cells including a chalcogenide material) may beoperated (e.g., read and/or programmed) by selecting respective accesslines (e.g., a bit line and a word line) and applying voltage or currentpulses.

While the example of FIG. 2 shows two memory decks, other configurationsare possible. In some examples, a single memory deck of self-selectingmemory cells may be constructed above a substrate 204, which may bereferred to as a two-dimensional memory. In some examples, a three orfour memory decks of memory cells may be configured in a similar mannerin a three-dimensional cross point architecture. In some examples, oneor more of the memory decks may include self-selecting memory cells thatinclude chalcogenide material 220. Chalcogenide material 220 may, forexample, include a chalcogenide glass such as, for example, an alloy ofselenium (Se), tellurium (Te), arsenic (As), antimony (Sb), carbon (C),germanium (Ge), and silicon (Si). In some examples, a chalcogenidematerial having primarily selenium (Se), arsenic (As), and germanium(Ge) may be referred to as SAG-alloy. In some examples, SAG-alloy mayinclude silicon (Si) and such chalcogenide material may be referred toas SiSAG-alloy. In some examples, the chalcogenide glass may includeadditional elements such as hydrogen (H), oxygen (O), nitrogen (N),chlorine (Cl), or fluorine (F), each in atomic or molecular forms.

In some examples, a self-selecting memory cell including chalcogenidematerial 220 may be accessed by applying pre-read voltages and one ormore read voltages to the self-selecting memory cell using a bit line115 and a word line 110. In one example, a controller associated with aself-selecting memory cell may apply a plurality of voltages (e.g.,pre-read voltages) to a plurality of self-selecting memory cells. Basedon a distribution identified from applying the plurality of voltages, afirst read voltage may be determined and applied to the self-selectingmemory cell. A second read voltage may be determined based on thedetermined first read voltage, and applied to the self-selecting memorycell. In such cases, a controller associated with the self-selectingmemory cell may determine the logic state stored by the memory cellbased on applying the first read voltage and the second read voltage.

FIG. 3A illustrates an example of a diagram 300 showing distributions ofthreshold voltages of a self-selecting memory cell that supports thatsupports accessing a multi-level memory cell in accordance with examplesas disclosed herein. A multi-level self-selecting memory cell may beconfigured to store a logic state that represents multiple bits of datausing a multi-level storage techniques. The voltage distributions depictlogic states that may be read. The threshold voltage distributions mayrepresent a multi-level cell programming scheme for accessing themulti-level memory cell.

In the example of FIG. 3A, the distribution 305 may represent a setstate (e.g., first logic state), the distribution 310 may represent anintermediate state (e.g., second logic state), and the distribution 315may represent a reset state (e.g., third logic state). In some cases,the distributions 305, 310, and 315 may exhibit a median voltage value(such as a normal quantile) corresponding to a voltage distribution foreach logic state. In some examples, two distributions may have anoverlapping portion, thus may not have clear separation between the twodistributions. In some examples, each distribution may not besymmetrical around its median. In some examples, each distribution mayexhibit a different ranges of voltage values. In some cases, theintermediate state (e.g., distribution 310) may be programmed withvoltage having a same polarity as the set state (e.g., distribution 305)or the reset state (e.g., distribution 315). In some cases, the diagram300 may include distributions that represent more than three logicstates. For example, the diagram 300 may include distributions thatrepresent a fourth logic state, a fifth logic state, or more logicstates.

FIG. 3B illustrates an example of a timing diagram 360 that supportsaccessing a multi-level memory cell in accordance with examples asdisclosed herein. The timing diagram 360 may include a pre-read portion320 and a read portion 325. The timing diagram 360 may plot a magnitudeof the voltage of one or more pulses (y-axis) applied to the memory cellwith respect to time (x-axis).

The pre-read portion 320 of the timing diagram 360 may include aplurality of voltages 330. The plurality of voltages 330 may be anexample of pre-read voltages. During the pre-read portion 320, theplurality of voltages 330 may be applied to different sets of memorycells. For example, the memory device may apply the plurality ofvoltages 330 to a plurality of memory cells as part of a pre-readportion 320 of a read operation. The pre-read portion of the readoperation may be used to determine a distribution for at least one logicstate stored by the memory cells. Over time, the voltage thresholdsassociated with a first logic state stored by memory cells may driftbased on changing conditions in the memory device. Using thedistribution determined during the pre-read portion, read voltages maybe selected that may reduce or mitigate a likelihood of errors duringthe read operation.

The plurality of voltage 330 may include at least a first voltage 335-a,a second voltage 335-b, and a third voltage 335-c. The plurality ofvoltages 330 may be a same polarity as the first read voltage 340. Insome cases, the magnitude of each of the plurality of voltages 330 maybe different than a magnitude 345 of the first read voltage 340. Forexample, the magnitude of each of the first voltage 335-a, the secondvoltage 335-b, and the third voltage 335-c may be less than themagnitude 345 of the first read voltage 340.

In some cases, the pre-read portion 320 may include applying differentvoltages (e.g., the plurality of voltages 330) to one or more partitionsof the memory array. The memory array may include a plurality ofpartitions where each partition may include the plurality of memorycells (e.g., two or more memory cells). As part of the pre-read portion320, the memory device may apply a first voltage 335-a to a firstpartition of the plurality of memory cells and identify a first quantityof memory cells of the first partition in which a snapback event occurs.The memory device may then apply a second voltage 335-b to a secondpartition of the plurality of memory cells and identify a secondquantity of memory cells of the second partition in which the snapbackevent occurs. The memory device may apply a third voltage 335-c to athird partition of the plurality of memory cells and identify a thirdquantity of memory cells of the third partition in which the snapbackevent occurs. Based on the first quantity, the second quantity, and thethird quantity of memory cells, the memory device may identify amagnitude 345 or a polarity or both of the first read voltage 340, asdescribed below in further detail.

In some examples, the memory device may apply any quantity of theplurality of voltage 330 to any quantity memory cells or partitions. Forexample, the memory device may apply different voltages 330 to differentpartitions of the plurality of memory cells and identify quantities ofmemory cells in which snapback events occur. In such cases, the memorydevice may identify the magnitude 345 or a polarity or both of the firstread voltage 340 based on the quantities of memory cells. In some cases,the memory device may apply any quantity of the plurality of voltages330 to a sub-bank or bits of a memory device.

In some examples, the plurality of voltages 330 may be applied tocodewords of the partitions. The codewords may be divided into pools ofcodewords such that each pool of codewords may be read at differentvoltages 330. Based on applying the plurality of voltages 330 tocodewords, the memory device may then identify a likely distribution forone of the logic states in the memory cells. For example, the memorydevice may combine the pre-read data (e.g., data associated withapplying the first voltage 335-a, the second voltage 335-b, and thethird voltage 335-c) to determine a shape of the distribution. In suchcases, the memory device may aggregate data associated with applying thefirst voltage 335-a to the first partition, applying the second voltage335-b to the second partition, and applying the third voltage 335-c tothe third partition. The aggregation of pre-read data may improve theefficiency of the memory device by reducing the read latency andselecting the first read voltage based on the pre-read data. In somecases, the memory device may determine a shape of the distribution byidentifying one or more likely distributions of threshold voltagesassociated with memory cells that store a first logic state (e.g.,distribution 305).

The shape of the distribution may be determined based on reading thememory cells at different voltages 330 (e.g., first voltage 335-a, thesecond voltage 335-b, and the third voltage 335-c) and determining avoltage drift associated with each distribution (e.g., distribution 305,310, and 315). The memory device may ramp different partitions todifferent voltages 330 and determine an amount of bits thresholding ateach of the plurality of voltages 330 (e.g., first voltage 335-a, thesecond voltage 335-b, and the third voltage 335-c). An increased amountof bits thresholding may increase the accuracy of the determined shapeof the distribution. In some cases, determining that the amount of bitsthresholding may be above a threshold may identify that a snapback eventoccurred.

In some cases, a distribution that experiences a low or no voltage driftmay be placed at a lower priority level as compared to a distributionthat experiences a high of voltage drift. For example, distribution 305may experience little to no voltage drift while distribution 310 mayexperience a higher voltage drift as compared to distribution 305. Basedon the voltage drift, the first read voltage 340 may be selected betweenthe distribution that experiences the small or no voltage drift and thedistribution that experience the high voltage drift. The memory devicemay rank the distributions and select the highest priority as a startingpoint for the first read voltage 340.

The timing diagram 360 may include a read portion 325 after the pre-readportion 320. The read portion 325 may include a first read voltage 340and a second read voltage 350. Using that information associated withthe likely distribution and the shape of the distribution, the memorydevice may select the first read voltage 340 for the read portion 325 ofthe operation. In some examples, the memory device may count a quantityof set bits in the codeword by pre-reading the set state (e.g.,distribution 305). The first read voltage 340 may be determined based onaggregating the pre-read data, identifying the one or more likelydistributions of the threshold voltages, or both. In some examples, themagnitude 345 or a polarity or both of the first read voltage 340 may bedetermined based on the data associated with the pre-read portion 320.

The memory device may apply the first read voltage 340 to a memory cellof the plurality of memory cells to identify a logic state stored by thememory cell as part of the read portion 325 of the read operation. Insome cases, the memory device may identify a magnitude 345 of the firstread voltage 340 based on applying the plurality of voltages 330. Thememory device may also determine a polarity of the first read voltage340. For example, the memory device may determine that the polarity ofthe first read voltage 340 is a positive polarity.

In some cases, the first read voltage 340 may be between a voltageassociated with the distribution 305 and a voltage associated with thedistribution 310. In such cases, the memory device may determine thatthe first read voltage 340 is between a voltage of a first type of state(e.g., distribution 305) and between a voltage of a second type of state(e.g., distribution 310). The first type of state may be capable ofbeing stored by the memory cell that is disturbed after a snapback eventoccurs. The second type of state may be capable of being stored by thememory cell that is reinforced after the snapback event occurs. In someexamples, the memory device may read the logic state associated with thedistribution 305 based on applying the first read voltage 340.

A read disturbance may be induced after the application of the firstread voltage 340. In such cases, a write-back operation may be performedon the memory cell to reduce the read disturbance on an intermediatestate of the memory cell (e.g., distribution 310). The memory device maydetect the intermediate state associated with distribution 310 anddetermine whether a first snapback event occurred based on applying thefirst read voltage 340. The memory device may then perform a reprogramoperation on the memory cell after determining the logic state stored bythe memory cell is an intermediate state and determining that the firstsnapback event occurred. In some examples, the memory device maydetermine that the logic state associated with the distribution 315failed to experience a snapback event, thereby avoiding a readdisturbance on the distribution 315.

In some examples, the memory device may identify a voltage drift fromapplying the first read voltage 340. In such cases, the memory devicemay select a second read voltage 350 to apply to the memory cell. Forexample, the memory device may select the second read voltage 350 basedon an offset (e.g., a preconfigured or predetermined offset) between thefirst read voltage 340 and the second read voltage 350. In such cases,the memory device may identify an offset between the magnitude 345 ofthe first read voltage 340 and a magnitude 355 of the second readvoltage 350. Such an offset may be used because, in some cases, changesin the distribution of voltage thresholds of the first logic state maybe similar to changes in the distributions of other voltage thresholdsof other logic states.

In some cases, the memory device may determine that an amount of voltagedrift for the distribution 305 may be the same as an amount of voltagedrift for the distribution 310. In such cases, the memory device mayselect the second read voltage 350 based on the constant offset (e.g.,the amount of voltage drift). The voltage drift of distribution 315 maybe greater than the voltage drift of distributions 310 and 305. In suchcases, the sense window of distribution 315 may increase. The memorydevice may determine the magnitude 355 of the second read voltage 350based on determining the magnitude 345 of the first read voltage,determining the offset, or both. For example, the magnitude 355 of thesecond read voltage 350 may be greater than the magnitude 345 of thefirst read voltage 340.

After applying the first read voltage 340, the memory device may applythe second read voltage 350 to the memory cell of the plurality ofmemory cells as part of the read portion 325 of the read operation. Thesecond read voltage 350 may be between a voltage associated with thedistribution 310 and a voltage associated with the distribution 315. Insuch cases, the memory device may determine that the second read voltage350 is between a voltage of the second type of state (e.g., distribution310) and a voltage of a third type of state (e.g., distribution 315).The second type may be capable of being stored by the memory cell thatis reinforced after a snapback event occurs. The third type may becapable of being stored by the memory cell that is disturbed after thesnapback event occurs.

In some cases, the memory device may determine a polarity of the secondread voltage 350. For example, the memory device may determine that thepolarity of the second read voltage 350 is a positive polarity. In suchcases, the polarity of the second read voltage 350 is the same as thepolarity of the first read voltage 340. In some cases, applying thesecond read voltage 350 may be based on the determining the second readvoltage 350, identifying the polarity of the second read voltage 350,identifying the magnitude 355 of the second read voltage 350, or acombination thereof. In some examples, applying the second read voltage350 may be based on determining that the first snapback event failed tooccur.

The performance of the memory cell may increase by applying two readvoltages with a same polarity, thereby preventing the memory device fromchanging the polarity of the read voltage during the read portion 325.Changing polarities of voltages during a read operation may increase aduration of the read operation or may increase a power consumed by theread operation or both. In some cases, including, in the read operation,a pre-read portion 320 that identifies the first read voltage 340 of theread portion 325 may increase an amount of bits accessed per memorycell, reduce a size of the memory die, and decrease a density of thememory array, thereby reducing power consumption and increasing read,write, and erase operations of the memory device.

In some examples, devices, systems, and techniques are described foraccessing a multi-level self-selecting memory cell that stores more thanthree states. As described with reference to FIG. 3B, the memory devicemay select the second read voltage 350 to distinguish between storingthe second logic state and a third logic state based on the first readvoltage 340. In some cases, the memory device may select a third readvoltage to distinguish between storing the third logic state and afourth logic state based on the second read voltage 350. In otherexamples, the memory device may select a fourth read voltage todistinguish between storing the fourth logic state and a fifth logicstate based on the third read voltage. In such cases, the memory devicemay determine the fourth or fifth logic state based on applying thethird read voltage and the fourth read voltage.

FIG. 4A illustrates an example of a diagram 400 showing distributions ofthreshold voltages of a self-selecting memory cell that supports thatsupports accessing a multi-level memory cell in accordance with examplesas disclosed herein. Diagram 400 may be an example of diagram 300described with reference to FIG. 3A. In some cases, the diagram 400 mayinclude distributions that represent more than three logic states. Forexample, the diagram 400 may include distributions that represent afourth logic state, a fifth logic state, or more logic states.

FIG. 4B illustrates an example of a timing diagram 460 that supportsaccessing a multi-level memory cell in accordance with examples asdisclosed herein. The timing diagram 460 may include a pre-read portion420 and a read portion 425. The timing diagram 460 may plot a magnitudeof the voltage of one or more pulses (y-axis) applied to the memory cellwith respect to time (x-axis).

The pre-read portion 420 and the plurality of voltages 430 including thefirst voltage 435-a, second voltage 435-b, and third voltage 435-c mayeach be an example of pre-read portion 320, plurality of voltages 330,first voltage 335-a, second voltage 335-b, and third voltage 335-c,respectively, described with reference to FIG. 3B. The operationsperformed by the memory device and associated with the pre-read portion420 may be examples of the operations performed by the memory device andassociated with the pre-read portion 320 described with reference toFIG. 3B. The first read voltage 440 having a magnitude 445 may beexamples of the first read voltage 340 having the magnitude 345described with reference to FIG. 3B.

In some cases, the memory device may determine a polarity of the secondread voltage 450. For example, the memory device may determine that thepolarity of the second read voltage 450 is a negative polarity. In suchcases, the polarity of the second read voltage 450 is different thanpolarity of the first read voltage 440. The memory device may apply thesecond read voltage 450 with a second polarity to the memory cell aspart of the read portion 425 of the read operation.

The memory device may determine that a snapback event occurred at theset state associated with the distribution 405. In such cases, thememory device may flip the polarity of the reset state associated withthe distribution 415 and determine that a snapback event occurred at thereset state. In such cases, the set state and reset states may bereinforced after the snapback event occurs (e.g., the memory cell may berefreshed after determining that the snapback event occurs). Thepolarity of the first read voltage 440 may be selected to reinforce theset state (e.g., associated with distribution 405), and the polarity ofthe second read voltage 450 may be selected to reinforce the reset state(e.g., associated with distribution 415).

The intermediate state associated with the distribution 410 may remainundisturbed, thereby reducing an amount of write-back operationsperformed on the intermediate state as compared to the amount ofwrite-back operations described with reference to FIG. 3B. The memorydevice may determine that a snapback event failed to occur for theintermediate state. In such cases, the memory device may determine avoltage drift associated with the intermediate state based on a lack ofsnapback events and a lack of refresh operations. The read operationaccording to timing diagram 460 may be selected to reduce readdisturbance, while the read operation according to timing diagram 360may be selected to reduce voltage drift.

The memory device may change the polarity of the second read voltage 450(e.g., to be opposite the polarity of the first read voltage 440) duringthe read portion 425, by applying two sequential read voltages withopposite polarities. In some examples, the difference in polaritiesbetween the first read voltage 440 and the second read voltage 450 mayreduce an amount of write-back operations performed on the intermediatestate (e.g., distribution 410). In some cases, no write-back operationsmay be performed on the intermediate state based on using sequentialread voltages of opposite polarity. In such cases, the memory device maynot disturb the intermediate state in the positive polarity or thenegative polarity, thereby decreasing the power consumption andimproving the efficiency of the read operation as compared to the readoperation described with reference to FIG. 3B. Because the intermediatestate is not disturbed by the first read voltage or the second readvoltage, a write-back operation for the intermediate state may not beused, thereby reducing the duration of the read operation and the powerconsumption.

In some examples, devices, systems, and techniques are described foraccessing a multi-level self-selecting memory cell that stores more thanthree states. As described with reference to FIG. 4B, the memory devicemay select the second read voltage 450 to distinguish between storingthe second logic state and a third logic state based on the first readvoltage 440. In some cases, the memory device may select a third readvoltage to distinguish between storing the third logic state and afourth logic state based on the second read voltage 450. In otherexamples, the memory device may select a fourth read voltage todistinguish between storing the fourth logic state and a fifth logicstate based on the third read voltage. In such cases, the memory devicemay determine the fourth or fifth logic state based on applying thethird read voltage and the fourth read voltage.

FIG. 5 shows a block diagram 500 of a memory device 505 that supportsaccessing a multi-level memory cell in accordance with examples asdisclosed herein. The memory device 505 may be an example of aspects ofa memory device as described with reference to FIGS. 1 through 4 . Thememory device 505 may include a pre-read component 510, a first voltagecomponent 515, a second voltage component 520, a logic state component525, and a partition component 530. Each of these modules maycommunicate, directly or indirectly, with one another (e.g., via one ormore buses).

The pre-read component 510 may apply a set of voltages to a set ofmemory cells as part of a pre-read portion of a read operation. In someexamples, the pre-read component 510 may apply a first voltage to afirst partition of the set of memory cells. In some examples, thepre-read component 510 may apply a second voltage to a second partitionof the set of memory cells.

In some examples, the pre-read component 510 may identify a magnitude ora polarity or both of the first read voltage based on the first quantityand the second quantity. In some examples, the pre-read component 510may aggregate data associated with applying the first voltage to thefirst partition and applying the second voltage to the second partition,where identifying a first magnitude of the first read voltage is basedon aggregating the data.

In some examples, the pre-read component 510 may identify one or morelikely distributions of threshold voltages associated with memory cellsthat store a first logic state based on the first quantity and thesecond quantity, where identifying the magnitude or the polarity or bothof the first read voltage is based on identifying the one or more likelydistributions of threshold voltages. In some examples, the pre-readcomponent 510 may apply different voltages to different partitions ofthe set of memory cells. In some examples, the pre-read component 510may identify a magnitude or a polarity or both of the first read voltagebased on the quantities of memory cells.

The first voltage component 515 may apply a first read voltage to amemory cell of the set of memory cells to identify a logic state storedby the memory cell as part of a read portion of the read operation basedon applying the set of voltages. In some examples, the first voltagecomponent 515 may apply a first read voltage with a first magnitude anda first polarity to a memory cell of the set of memory cells to identifya logic state stored by the memory cell as part of a read portion of theread operation based on applying the set of voltages. In some examples,the first voltage component 515 may apply a first read voltage with afirst polarity to a memory cell of the set of memory cells to identify alogic state stored by the memory cell as part of a read portion of theread operation based on applying the set of voltages.

In some examples, the first voltage component 515 may identify a firstmagnitude of the first read voltage based on applying the set ofvoltages, where applying the first read voltage is based on identifyingthe first magnitude of the first read voltage. In some examples, thefirst voltage component 515 may determine that the first read voltage isbetween a voltage of a first type of state capable of being stored bythe memory cell that is disturbed after a snapback event occurs and avoltage of a second type of state capable of being stored by the memorycell that is reinforced after the snapback event occurs, where applyingthe first read voltage is based on the determining.

In some examples, the first voltage component 515 may identify apolarity of the first read voltage, where applying the first readvoltage is based on identifying the polarity of the first read voltage.In some examples, the first voltage component 515 may identify the firstpolarity of the first read voltage and the second read voltage, whereapplying the first read voltage and applying the second read voltage isbased on identifying the first polarity, where the first polarity is apositive polarity. In some examples, the first voltage component 515 mayidentify the first polarity of the first read voltage, where applyingthe first read voltage is based on identifying the first polarity of thefirst read voltage.

The second voltage component 520 may apply a second read voltage to thememory cell of the set of memory cells as part of the read portion ofthe read operation based on applying the first read voltage. In someexamples, the second voltage component 520 may apply a second readvoltage with a second magnitude and the first polarity to the memorycell as part of the read portion of the read operation based on applyingthe first read voltage. In some examples, the second voltage component520 may apply a second read voltage with a second polarity to the memorycell as part of the read portion of the read operation based on applyingthe first read voltage.

In some examples, the second voltage component 520 may identify a secondmagnitude of the second read voltage based on identifying the firstmagnitude of the first read voltage, where applying the second readvoltage is based on identifying the first magnitude of the second readvoltage. In some examples, the second voltage component 520 may identifythe second magnitude of the second read voltage based on identifying thefirst magnitude of the first read voltage, where applying the secondread voltage is based on identifying the first magnitude of the secondread voltage, where the first magnitude is less than the secondmagnitude. In some examples, the second voltage component 520 mayidentify a second magnitude of the second read voltage based onidentifying the first magnitude of the first read voltage, whereapplying the second read voltage is based on identifying the firstmagnitude of the second read voltage, where the first magnitude is equalto the second magnitude.

In some examples, the second voltage component 520 may identify anoffset between the first magnitude of the first read voltage and thesecond magnitude of the second read voltage, where identifying thesecond magnitude of the second read voltage is based on the offset andthe first magnitude. In some examples, the second voltage component 520may determine that the second read voltage is between a voltage of asecond type of state capable of being stored by the memory cell that isreinforced after a snapback event occurs and a voltage of a third typeof state capable of being stored by the memory cell that is disturbedafter the snapback event occurs, where applying the second read voltageis based on the determining.

In some examples, the second voltage component 520 may identify apolarity of the second read voltage, where applying the second readvoltage is based on identifying the polarity of the second read voltage.In some examples, the second voltage component 520 may identify thesecond polarity of the second read voltage, where applying the secondread voltage is based on identifying the second polarity of the secondread voltage, where the first polarity is a positive polarity and thesecond polarity is a negative polarity.

The logic state component 525 may determine the logic state stored bythe memory cell based on applying the first read voltage and applyingthe second read voltage. In some examples, the logic state component 525may determine whether a first snapback event occurred based on applyingthe first read voltage, where applying the second read voltage is basedon determining that the first snapback event failed to occur. In someexamples, the logic state component 525 may perform a reprogramoperation on the memory cell after determining the logic state stored bythe memory cell based on determining that the first snapback eventoccurred.

The partition component 530 may identify a first quantity of memorycells of the first partition in which a snapback event occurs based onapplying the first voltage. In some examples, the partition component530 may identify a second quantity of memory cells of the secondpartition in which the snapback event occurs based on applying thesecond voltage. In some examples, the partition component 530 mayidentify quantities of memory cells in which snapback events occur basedon applying the different voltages.

FIG. 6 shows a flowchart illustrating a method or methods 600 thatsupports accessing a multi-level memory cell in accordance with examplesas disclosed herein. The operations of method 600 may be implemented bya memory device or its components as described herein. For example, theoperations of method 600 may be performed by a memory device asdescribed with reference to FIG. 5 . In some examples, a memory devicemay execute a set of instructions to control the functional elements ofthe memory device to perform the described functions. Additionally oralternatively, a memory device may perform aspects of the describedfunctions using special-purpose hardware.

At 605, the memory device may apply a set of voltages to a set of memorycells as part of a pre-read portion of a read operation. The operationsof 605 may be performed according to the methods described herein. Insome examples, aspects of the operations of 605 may be performed by apre-read component as described with reference to FIG. 5 .

At 610, the memory device may apply a first read voltage to a memorycell of the set of memory cells to identify a logic state stored by thememory cell as part of a read portion of the read operation based onapplying the set of voltages. The operations of 610 may be performedaccording to the methods described herein. In some examples, aspects ofthe operations of 610 may be performed by a first voltage component asdescribed with reference to FIG. 5 .

At 615, the memory device may apply a second read voltage to the memorycell of the set of memory cells as part of the read portion of the readoperation based on applying the first read voltage. The operations of615 may be performed according to the methods described herein. In someexamples, aspects of the operations of 615 may be performed by a secondvoltage component as described with reference to FIG. 5 .

At 620, the memory device may determine the logic state stored by thememory cell based on applying the first read voltage and applying thesecond read voltage. The operations of 620 may be performed according tothe methods described herein. In some examples, aspects of theoperations of 620 may be performed by a logic state component asdescribed with reference to FIG. 5 .

In some examples, an apparatus as described herein may perform a methodor methods, such as the method 600. The apparatus may include features,means, or instructions (e.g., a non-transitory computer-readable mediumstoring instructions executable by a processor) for applying a set ofvoltages to a set of memory cells as part of a pre-read portion of aread operation, applying a first read voltage to a memory cell of theset of memory cells to identify a logic state stored by the memory cellas part of a read portion of the read operation based on applying theset of voltages, applying a second read voltage to the memory cell ofthe set of memory cells as part of the read portion of the readoperation based on applying the first read voltage, and determining thelogic state stored by the memory cell based on applying the first readvoltage and applying the second read voltage.

Some examples of the method 600 and the apparatus described herein mayfurther include operations, features, means, or instructions foridentifying a first magnitude of the first read voltage based onapplying the set of voltages, where applying the first read voltage maybe based on identifying the first magnitude of the first read voltage.

Some examples of the method 600 and the apparatus described herein mayfurther include operations, features, means, or instructions foridentifying a second magnitude of the second read voltage based onidentifying the first magnitude of the first read voltage, whereapplying the second read voltage may be based on identifying the firstmagnitude of the second read voltage.

Some examples of the method 600 and the apparatus described herein mayfurther include operations, features, means, or instructions foridentifying an offset between the first magnitude of the first readvoltage and the second magnitude of the second read voltage, whereidentifying the second magnitude of the second read voltage may be basedon the offset and the first magnitude.

In some examples of the method 600 and the apparatus described herein,applying the set of voltages further may include operations, features,means, or instructions for applying a first voltage to a first partitionof the set of memory cells, identifying a first quantity of memory cellsof the first partition in which a snapback event occurs based onapplying the first voltage, applying a second voltage to a secondpartition of the set of memory cells, identifying a second quantity ofmemory cells of the second partition in which the snapback event occursbased on applying the second voltage, and identifying a magnitude or apolarity or both of the first read voltage based on the first quantityand the second quantity.

Some examples of the method 600 and the apparatus described herein mayfurther include operations, features, means, or instructions foraggregating data associated with applying the first voltage to the firstpartition and applying the second voltage to the second partition, whereidentifying a first magnitude of the first read voltage may be based onaggregating the data.

Some examples of the method 600 and the apparatus described herein mayfurther include operations, features, means, or instructions foridentifying one or more likely distributions of threshold voltagesassociated with memory cells that store a first logic state based on thefirst quantity and the second quantity, where identifying the magnitudeor the polarity or both of the first read voltage may be based onidentifying the one or more likely distributions of threshold voltages.

In some examples of the method 600 and the apparatus described herein,the first partition and the second partition each include two or morememory cells of the set of memory cells.

Some examples of the method 600 and the apparatus described herein mayfurther include operations, features, means, or instructions fordetermining that the first read voltage may be between a voltage of afirst type of state capable of being stored by the memory cell that maybe disturbed after a snapback event occurs and a voltage of a secondtype of state capable of being stored by the memory cell that may bereinforced after the snapback event occurs, where applying the firstread voltage may be based on the determining.

Some examples of the method 600 and the apparatus described herein mayfurther include operations, features, means, or instructions fordetermining that the second read voltage may be between a voltage of asecond type of state capable of being stored by the memory cell that maybe reinforced after a snapback event occurs and a voltage of a thirdtype of state capable of being stored by the memory cell that may bedisturbed after the snapback event occurs, where applying the secondread voltage may be based on the determining.

Some examples of the method 600 and the apparatus described herein mayfurther include operations, features, means, or instructions foridentifying a polarity of the first read voltage, where applying thefirst read voltage may be based on identifying the polarity of the firstread voltage, and identifying a polarity of the second read voltage,where applying the second read voltage may be based on identifying thepolarity of the second read voltage.

In some examples of the method 600 and the apparatus described herein,the polarity of the first read voltage may be the same as the polarityof the second read voltage.

In some examples of the method 600 and the apparatus described herein,the polarity of the first read voltage may be different than thepolarity of the second read voltage.

In some examples of the method 600 and the apparatus described herein,applying the set of voltages further may include operations, features,means, or instructions for applying different voltages to differentpartitions of the set of memory cells, identifying quantities of memorycells in which snapback events occur based on applying the differentvoltages, and identifying a magnitude or a polarity or both of the firstread voltage based on the quantities of memory cells.

Some examples of the method 600 and the apparatus described herein mayfurther include operations, features, means, or instructions fordetermining whether a first snapback event occurred based on applyingthe first read voltage, where applying the second read voltage may bebased on determining that the first snapback event failed to occur.

Some examples of the method 600 and the apparatus described herein mayfurther include operations, features, means, or instructions forperforming a reprogram operation on the memory cell after determiningthe logic state stored by the memory cell based on determining that thefirst snapback event occurred.

FIG. 7 shows a flowchart illustrating a method or methods 700 thatsupports accessing a multi-level memory cell in accordance with examplesas disclosed herein. The operations of method 700 may be implemented bya memory device or its components as described herein. For example, theoperations of method 700 may be performed by a memory device asdescribed with reference to FIG. 5 . In some examples, a memory devicemay execute a set of instructions to control the functional elements ofthe memory device to perform the described functions. Additionally oralternatively, a memory device may perform aspects of the describedfunctions using special-purpose hardware.

At 705, the memory device may apply a set of voltages to a set of memorycells as part of a pre-read portion of a read operation. The operationsof 705 may be performed according to the methods described herein. Insome examples, aspects of the operations of 705 may be performed by apre-read component as described with reference to FIG. 5 .

At 710, the memory device may apply a first read voltage with a firstmagnitude and a first polarity to a memory cell of the set of memorycells to identify a logic state stored by the memory cell as part of aread portion of the read operation based on applying the set ofvoltages. The operations of 710 may be performed according to themethods described herein. In some examples, aspects of the operations of710 may be performed by a first voltage component as described withreference to FIG. 5 .

At 715, the memory device may apply a second read voltage with a secondmagnitude and the first polarity to the memory cell as part of the readportion of the read operation based on applying the first read voltage.The operations of 715 may be performed according to the methodsdescribed herein. In some examples, aspects of the operations of 715 maybe performed by a second voltage component as described with referenceto FIG. 5 .

At 720, the memory device may determine the logic state stored by thememory cell based on applying the first read voltage and applying thesecond read voltage. The operations of 720 may be performed according tothe methods described herein. In some examples, aspects of theoperations of 720 may be performed by a logic state component asdescribed with reference to FIG. 5 .

In some examples, an apparatus as described herein may perform a methodor methods, such as the method 700. The apparatus may include features,means, or instructions (e.g., a non-transitory computer-readable mediumstoring instructions executable by a processor) for applying a set ofvoltages to a set of memory cells as part of a pre-read portion of aread operation, applying a first read voltage with a first magnitude anda first polarity to a memory cell of the set of memory cells to identifya logic state stored by the memory cell as part of a read portion of theread operation based on applying the set of voltages, applying a secondread voltage with a second magnitude and the first polarity to thememory cell as part of the read portion of the read operation based onapplying the first read voltage, and determining the logic state storedby the memory cell based on applying the first read voltage and applyingthe second read voltage.

Some examples of the method 700 and the apparatus described herein mayfurther include operations, features, means, or instructions foridentifying the first magnitude of the first read voltage based onapplying the set of voltages, where applying the first read voltage maybe based on identifying the first magnitude of the first read voltage.

Some examples of the method 700 and the apparatus described herein mayfurther include operations, features, means, or instructions foridentifying the second magnitude of the second read voltage based onidentifying the first magnitude of the first read voltage, whereapplying the second read voltage may be based on identifying the firstmagnitude of the second read voltage, where the first magnitude may beless than the second magnitude.

Some examples of the method 700 and the apparatus described herein mayfurther include operations, features, means, or instructions foridentifying an offset between the first magnitude of the first readvoltage and the second magnitude of the second read voltage, whereidentifying the second magnitude of the second read voltage may be basedon the offset and the first magnitude.

In some examples of the method 700 and the apparatus described herein,applying the set of voltages further may include operations, features,means, or instructions for applying a first voltage to a first partitionof the set of memory cells, identifying a first quantity of memory cellsof the first partition in which a snapback event occurs based onapplying the first voltage, applying a second voltage to a secondpartition of the set of memory cells, identifying a second quantity ofmemory cells of the second partition in which the snapback event occursbased on applying the second voltage, and identifying a magnitude or apolarity or both of the first read voltage based on the first quantityand the second quantity.

Some examples of the method 700 and the apparatus described herein mayfurther include operations, features, means, or instructions foridentifying the first polarity of the first read voltage and the secondread voltage, where applying the first read voltage and applying thesecond read voltage may be based on identifying the first polarity,where the first polarity may be a positive polarity.

FIG. 8 shows a flowchart illustrating a method or methods 800 thatsupports accessing a multi-level memory cell in accordance with examplesas disclosed herein. The operations of method 800 may be implemented bya memory device or its components as described herein. For example, theoperations of method 800 may be performed by a memory device asdescribed with reference to FIG. 5 . In some examples, a memory devicemay execute a set of instructions to control the functional elements ofthe memory device to perform the described functions. Additionally oralternatively, a memory device may perform aspects of the describedfunctions using special-purpose hardware.

At 805, the memory device may apply a set of voltages to a set of memorycells as part of a pre-read portion of a read operation. The operationsof 805 may be performed according to the methods described herein. Insome examples, aspects of the operations of 805 may be performed by apre-read component as described with reference to FIG. 5 .

At 810, the memory device may apply a first read voltage with a firstpolarity to a memory cell of the set of memory cells to identify a logicstate stored by the memory cell as part of a read portion of the readoperation based on applying the set of voltages. The operations of 810may be performed according to the methods described herein. In someexamples, aspects of the operations of 810 may be performed by a firstvoltage component as described with reference to FIG. 5 .

At 815, the memory device may apply a second read voltage with a secondpolarity to the memory cell as part of the read portion of the readoperation based on applying the first read voltage. The operations of815 may be performed according to the methods described herein. In someexamples, aspects of the operations of 815 may be performed by a secondvoltage component as described with reference to FIG. 5 .

At 820, the memory device may determine the logic state stored by thememory cell based on applying the first read voltage and applying thesecond read voltage. The operations of 820 may be performed according tothe methods described herein. In some examples, aspects of theoperations of 820 may be performed by a logic state component asdescribed with reference to FIG. 5 .

In some examples, an apparatus as described herein may perform a methodor methods, such as the method 800. The apparatus may include features,means, or instructions (e.g., a non-transitory computer-readable mediumstoring instructions executable by a processor) for applying a set ofvoltages to a set of memory cells as part of a pre-read portion of aread operation, applying a first read voltage with a first polarity to amemory cell of the set of memory cells to identify a logic state storedby the memory cell as part of a read portion of the read operation basedon applying the set of voltages, applying a second read voltage with asecond polarity to the memory cell as part of the read portion of theread operation based on applying the first read voltage, and determiningthe logic state stored by the memory cell based on applying the firstread voltage and applying the second read voltage.

Some examples of the method 800 and the apparatus described herein mayfurther include operations, features, means, or instructions foridentifying a first magnitude of the first read voltage based onapplying the set of voltages, where applying the first read voltage maybe based on identifying the first magnitude of the first read voltage.

Some examples of the method 800 and the apparatus described herein mayfurther include operations, features, means, or instructions foridentifying a second magnitude of the second read voltage based onidentifying the first magnitude of the first read voltage, whereapplying the second read voltage may be based on identifying the firstmagnitude of the second read voltage, where the first magnitude may beequal to the second magnitude.

Some examples of the method 800 and the apparatus described herein mayfurther include operations, features, means, or instructions foridentifying an offset between the first magnitude of the first readvoltage and the second magnitude of the second read voltage, whereidentifying the second magnitude of the second read voltage may be basedon the offset and the first magnitude.

In some examples of the method 800 and the apparatus described herein,applying the set of voltages further may include operations, features,means, or instructions for applying a first voltage to a first partitionof the set of memory cells, identifying a first quantity of memory cellsof the first partition in which a snapback event occurs based onapplying the first voltage, applying a second voltage to a secondpartition of the set of memory cells, identifying a second quantity ofmemory cells of the second partition in which the snapback event occursbased on applying the second voltage, and identifying a magnitude or apolarity or both of the first read voltage based on the first quantityand the second quantity.

Some examples of the method 800 and the apparatus described herein mayfurther include operations, features, means, or instructions foridentifying the first polarity of the first read voltage, where applyingthe first read voltage may be based on identifying the first polarity ofthe first read voltage, and identifying the second polarity of thesecond read voltage, where applying the second read voltage may be basedon identifying the second polarity of the second read voltage, where thefirst polarity may be a positive polarity and the second polarity may bea negative polarity.

It should be noted that the methods described herein are possibleimplementations, and that the operations and the steps may be rearrangedor otherwise modified and that other implementations are possible.Furthermore, portions from two or more of the methods may be combined.

An apparatus is described. The apparatus may include a memory arraycomprising a set of memory cells and a control component coupled withthe memory array, the control component configured to cause theapparatus to apply a set of voltages to a set of memory cells as part ofa pre-read portion of a read operation, apply a first read voltage to amemory cell of the set of memory cells to identify a logic state storedby the memory cell as part of a read portion of the read operation basedon applying the set of voltages, apply a second read voltage to thememory cell of the set of memory cells as part of the read portion ofthe read operation based on applying the first read voltage, anddetermine the logic state stored by the memory cell based on applyingthe first read voltage and applying the second read voltage.

Some examples may further include identifying a first magnitude of thefirst read voltage based on applying the set of voltages, where applyingthe first read voltage may be based on identifying the first magnitudeof the first read voltage.

Some examples may further include identifying a second magnitude of thesecond read voltage based on identifying the first magnitude of thefirst read voltage, where applying the second read voltage may be basedon identifying the first magnitude of the second read voltage.

Some examples may further include identifying an offset between thefirst magnitude of the first read voltage and the second magnitude ofthe second read voltage, where identifying the second magnitude of thesecond read voltage may be based on the offset and the first magnitude.

Some examples may further include applying a first voltage to a firstpartition of the set of memory cells, identifying a first quantity ofmemory cells of the first partition in which a snapback event occursbased on applying the first voltage, apply a second voltage to a secondpartition of the set of memory cells, identifying a second quantity ofmemory cells of the second partition in which the snapback event occursbased on applying the second voltage, and identifying a magnitude or apolarity or both of the first read voltage based on the first quantityand the second quantity.

Some examples may further include aggregating data associated withapplying the first voltage to the first partition and applying thesecond voltage to the second partition, where identifying a firstmagnitude of the first read voltage may be based on aggregating thedata.

Some examples may further include identifying one or more likelydistributions of threshold voltages associated with memory cells thatstore a first logic state based on the first quantity and the secondquantity, where identifying the magnitude or the polarity or both of thefirst read voltage may be based on identifying the one or more likelydistributions of threshold voltages.

In some examples, the first partition and the second partition eachinclude two or more memory cells of the set of memory cells.

Some examples may further include determining that the first readvoltage may be between a voltage of a first type of state capable ofbeing stored by the memory cell that may be disturbed after a snapbackevent occurs and a voltage of a second type of state capable of beingstored by the memory cell that may be reinforced after the snapbackevent occurs, where applying the first read voltage may be based on thedetermining.

Some examples may further include determining that the second readvoltage may be between a voltage of a second type of state capable ofbeing stored by the memory cell that may be reinforced after a snapbackevent occurs and a voltage of a third type of state capable of beingstored by the memory cell that may be disturbed after the snapback eventoccurs, where applying the second read voltage may be based on thedetermining.

Some examples may further include identifying a polarity of the firstread voltage, where applying the first read voltage may be based onidentifying the polarity of the first read voltage, and identifying apolarity of the second read voltage, where applying the second readvoltage may be based on identifying the polarity of the second readvoltage.

In some examples, the polarity of the first read voltage may be the sameas the polarity of the second read voltage.

In some examples, the polarity of the first read voltage may bedifferent than the polarity of the second read voltage.

Some examples may further include applying different voltages todifferent partitions of the set of memory cells, identifying quantitiesof memory cells in which snapback events occur based on applying thedifferent voltages, and identifying a magnitude or a polarity or both ofthe first read voltage based on the quantities of memory cells.

Some examples may further include determining whether a first snapbackevent occurred based on applying the first read voltage, where applyingthe second read voltage may be based on determining that the firstsnapback event failed to occur.

Some examples may further include performing a reprogram operation onthe memory cell after determining the logic state stored by the memorycell based on determining that the first snapback event occurred.

An apparatus is described. The apparatus may include a memory arraycomprising a set of memory cells and a control component coupled withthe memory array, the control component configured to cause theapparatus to apply a set of voltages to a set of memory cells as part ofa pre-read portion of a read operation, apply a first read voltage witha first magnitude and a first polarity to a memory cell of the set ofmemory cells to identify a logic state stored by the memory cell as partof a read portion of the read operation based on applying the set ofvoltages, apply a second read voltage with a second magnitude and thefirst polarity to the memory cell as part of the read portion of theread operation based on applying the first read voltage, and determinethe logic state stored by the memory cell based on applying the firstread voltage and applying the second read voltage.

Some examples may further include identifying the first magnitude of thefirst read voltage based on applying the set of voltages, where applyingthe first read voltage may be based on identifying the first magnitudeof the first read voltage.

Some examples may further include identifying the second magnitude ofthe second read voltage based on identifying the first magnitude of thefirst read voltage, where applying the second read voltage may be basedon identifying the first magnitude of the second read voltage, where thefirst magnitude may be less than the second magnitude.

Some examples may further include identifying an offset between thefirst magnitude of the first read voltage and the second magnitude ofthe second read voltage, where identifying the second magnitude of thesecond read voltage may be based on the offset and the first magnitude.

Some examples may further include applying a first voltage to a firstpartition of the set of memory cells, identifying a first quantity ofmemory cells of the first partition in which a snapback event occursbased on applying the first voltage, applying a second voltage to asecond partition of the set of memory cells, identify a second quantityof memory cells of the second partition in which the snapback eventoccurs based on applying the second voltage, and identifying a magnitudeor a polarity or both of the first read voltage based on the firstquantity and the second quantity.

Some examples may further include identifying the first polarity of thefirst read voltage and the second read voltage, where applying the firstread voltage and applying the second read voltage may be based onidentifying the first polarity, where the first polarity may be apositive polarity.

An apparatus is described. The apparatus may include a memory arraycomprising a set of memory cells and a control component coupled withthe memory array, the control component configured to cause theapparatus to apply a set of voltages to a set of memory cells as part ofa pre-read portion of a read operation, apply a first read voltage witha first polarity to a memory cell of the set of memory cells to identifya logic state stored by the memory cell as part of a read portion of theread operation based on applying the set of voltages, apply a secondread voltage with a second polarity to the memory cell as part of theread portion of the read operation based on applying the first readvoltage, and determine the logic state stored by the memory cell basedon applying the first read voltage and applying the second read voltage.

Some examples may further include identifying a first magnitude of thefirst read voltage based on applying the set of voltages, where applyingthe first read voltage may be based on identifying the first magnitudeof the first read voltage.

Some examples may further include identifying a second magnitude of thesecond read voltage based on identifying the first magnitude of thefirst read voltage, where applying the second read voltage may be basedon identifying the first magnitude of the second read voltage, where thefirst magnitude may be equal to the second magnitude.

Some examples may further include identifying an offset between thefirst magnitude of the first read voltage and the second magnitude ofthe second read voltage, where identifying the second magnitude of thesecond read voltage may be based on the offset and the first magnitude.

Some examples may further include applying a first voltage to a firstpartition of the set of memory cells, identify a first quantity ofmemory cells of the first partition in which a snapback event occursbased on applying the first voltage, applying a second voltage to asecond partition of the set of memory cells, identifying a secondquantity of memory cells of the second partition in which the snapbackevent occurs based on applying the second voltage, and identifying amagnitude or a polarity or both of the first read voltage based on thefirst quantity and the second quantity.

Some examples may further include identifying the first polarity of thefirst read voltage, where applying the first read voltage may be basedon identifying the first polarity of the first read voltage, andidentifying the second polarity of the second read voltage, whereapplying the second read voltage may be based on identifying the secondpolarity of the second read voltage, where the first polarity may be apositive polarity and the second polarity may be a negative polarity.

Information and signals described herein may be represented using any ofa variety of different technologies and techniques. For example, data,instructions, commands, information, signals, bits, symbols, and chipsthat may be referenced throughout the above description may berepresented by voltages, currents, electromagnetic waves, magneticfields or particles, optical fields or particles, or any combinationthereof. Some drawings may illustrate signals as a single signal;however, it will be understood by a person of ordinary skill in the artthat the signal may represent a bus of signals, where the bus may have avariety of bit widths.

The terms “electronic communication,” “conductive contact,” “connected,”and “coupled” may refer to a relationship between components thatsupports the flow of signals between the components. Components areconsidered in electronic communication with (or in conductive contactwith or connected with or coupled with) one another if there is anyconductive path between the components that can, at any time, supportthe flow of signals between the components. At any given time, theconductive path between components that are in electronic communicationwith each other (or in conductive contact with or connected with orcoupled with) may be an open circuit or a closed circuit based on theoperation of the device that includes the connected components. Theconductive path between connected components may be a direct conductivepath between the components or the conductive path between connectedcomponents may be an indirect conductive path that may includeintermediate components, such as switches, transistors, or othercomponents. In some examples, the flow of signals between the connectedcomponents may be interrupted for a time, for example, using one or moreintermediate components such as switches or transistors.

As used herein, the term “electrode” may refer to an electricalconductor, and in some examples, may be employed as an electricalcontact to a memory cell or other component of a memory array. Anelectrode may include a trace, wire, conductive line, conductivematerial, or the like that provides a conductive path between elementsor components of the memory array.

The devices discussed herein, including a memory array, may be formed ona semiconductor substrate, such as silicon, germanium, silicon-germaniumalloy, gallium arsenide, gallium nitride, etc. In some examples, thesubstrate is a semiconductor wafer. In other cases, the substrate may bea silicon-on-insulator (SOI) substrate, such as silicon-on-glass (SOG)or silicon-on-sapphire (SOS), or epitaxial layers of semiconductormaterials on another substrate. The conductivity of the substrate, orsub-regions of the substrate, may be controlled through doping usingvarious chemical species including, but not limited to, phosphorous,boron, or arsenic. Doping may be performed during the initial formationor growth of the substrate, by ion-implantation, or by any other dopingmeans.

A switching component or a transistor discussed herein may represent afield-effect transistor (FET) and comprise a three terminal deviceincluding a source, drain, and gate. The terminals may be connected toother electronic elements through conductive materials, e.g., metals.The source and drain may be conductive and may comprise a heavily-doped,e.g., degenerate, semiconductor region. The source and drain may beseparated by a lightly-doped semiconductor region or channel. If thechannel is n-type (i.e., majority carriers are electrons), then the FETmay be referred to as a n-type FET. If the channel is p-type (i.e.,majority carriers are holes), then the FET may be referred to as ap-type FET. The channel may be capped by an insulating gate oxide. Thechannel conductivity may be controlled by applying a voltage to thegate. For example, applying a positive voltage or negative voltage to ann-type FET or a p-type FET, respectively, may result in the channelbecoming conductive. A transistor may be “on” or “activated” when avoltage greater than or equal to the transistor's threshold voltage isapplied to the transistor gate. The transistor may be “off” or“deactivated” when a voltage less than the transistor's thresholdvoltage is applied to the transistor gate.

The description set forth herein, in connection with the appendeddrawings, describes example configurations and does not represent allthe examples that may be implemented or that are within the scope of theclaims. The term “exemplary” used herein means “serving as an example,instance, or illustration,” and not “preferred” or “advantageous overother examples.” The detailed description includes specific details toproviding an understanding of the described techniques. Thesetechniques, however, may be practiced without these specific details. Insome instances, well-known structures and devices are shown in blockdiagram form to avoid obscuring the concepts of the described examples.

In the appended figures, similar components or features may have thesame reference label. Further, various components of the same type maybe distinguished by following the reference label by a dash and a secondlabel that distinguishes among the similar components. If just the firstreference label is used in the specification, the description isapplicable to any one of the similar components having the same firstreference label irrespective of the second reference label.

Information and signals described herein may be represented using any ofa variety of different technologies and techniques. For example, data,instructions, commands, information, signals, bits, symbols, and chipsthat may be referenced throughout the above description may berepresented by voltages, currents, electromagnetic waves, magneticfields or particles, optical fields or particles, or any combinationthereof.

The various illustrative blocks and modules described in connection withthe disclosure herein may be implemented or performed with ageneral-purpose processor, a DSP, an ASIC, an FPGA or other programmablelogic device, discrete gate or transistor logic, discrete hardwarecomponents, or any combination thereof designed to perform the functionsdescribed herein. A general-purpose processor may be a microprocessor,but in the alternative, the processor may be any processor, controller,microcontroller, or state machine. A processor may also be implementedas a combination of computing devices (e.g., a combination of a DSP anda microprocessor, multiple microprocessors, one or more microprocessorsin conjunction with a DSP core, or any other such configuration).

The functions described herein may be implemented in hardware, softwareexecuted by a processor, firmware, or any combination thereof. Ifimplemented in software executed by a processor, the functions may bestored on or transmitted over as one or more instructions or code on acomputer-readable medium. Other examples and implementations are withinthe scope of the disclosure and appended claims. For example, due to thenature of software, functions described above can be implemented usingsoftware executed by a processor, hardware, firmware, hardwiring, orcombinations of any of these. Features implementing functions may alsobe physically located at various positions, including being distributedsuch that portions of functions are implemented at different physicallocations. Also, as used herein, including in the claims, “or” as usedin a list of items (for example, a list of items prefaced by a phrasesuch as “at least one of” or “one or more of”) indicates an inclusivelist such that, for example, a list of at least one of A, B, or C meansA or B or C or AB or AC or BC or ABC (i.e., A and B and C). Also, asused herein, the phrase “based on” shall not be construed as a referenceto a closed set of conditions. For example, an exemplary step that isdescribed as “based on condition A” may be based on both a condition Aand a condition B without departing from the scope of the presentdisclosure. In other words, as used herein, the phrase “based on” shallbe construed in the same manner as the phrase “based at least in parton.”

Computer-readable media includes both non-transitory computer storagemedia and communication media including any medium that facilitatestransfer of a computer program from one place to another. Anon-transitory storage medium may be any available medium that can beaccessed by a general purpose or special purpose computer. By way ofexample, and not limitation, non-transitory computer-readable media cancomprise RAM, ROM, electrically erasable programmable read-only memory(EEPROM), compact disk (CD) ROM or other optical disk storage, magneticdisk storage or other magnetic storage devices, or any othernon-transitory medium that can be used to carry or store desired programcode means in the form of instructions or data structures and that canbe accessed by a general-purpose or special-purpose computer, or ageneral-purpose or special-purpose processor. Also, any connection isproperly termed a computer-readable medium. For example, if the softwareis transmitted from a website, server, or other remote source using acoaxial cable, fiber optic cable, twisted pair, digital subscriber line(DSL), or wireless technologies such as infrared, radio, and microwave,then the coaxial cable, fiber optic cable, twisted pair, digitalsubscriber line (DSL), or wireless technologies such as infrared, radio,and microwave are included in the definition of medium. Disk and disc,as used herein, include CD, laser disc, optical disc, digital versatiledisc (DVD), floppy disk and Blu-ray disc where disks usually reproducedata magnetically, while discs reproduce data optically with lasers.Combinations of the above are also included within the scope ofcomputer-readable media.

The description herein is provided to enable a person skilled in the artto make or use the disclosure. Various modifications to the disclosurewill be apparent to those skilled in the art, and the generic principlesdefined herein may be applied to other variations without departing fromthe scope of the disclosure. Thus, the disclosure is not limited to theexamples and designs described herein but is to be accorded the broadestscope consistent with the principles and novel features disclosedherein.

What is claimed is:
 1. A method, comprising: applying a plurality ofvoltages as part of a pre-read portion of a read operation; applying afirst read voltage to a memory cell as part of a read portion of theread operation based at least in part on applying the plurality ofvoltages; identifying a second read voltage that comprises a differentpolarity than the first read voltage based at least in part on applyingthe first read voltage; and identifying a logic state stored by thememory cell based at least in part on applying the first read voltageand the second read voltage.
 2. The method of claim 1, wherein applyingthe second read voltage to the memory cell is done as part of the readportion of the read operation based at least in part on applying thefirst read voltage.
 3. The method of claim 2, further comprising:identifying that the second read voltage comprises a same polarity asthe first read voltage, wherein applying the second read voltage isbased at least in part on identifying that the second read voltagecomprises the same polarity.
 4. The method of claim 2, wherein applyingthe second read voltage is based at least in part on identifying thatthe second read voltage comprises the different polarity.
 5. The methodof claim 2, further comprising: identifying that the second read voltagecomprises a different magnitude as the first read voltage, whereinapplying the second read voltage is based at least in part onidentifying that the second read voltage comprises the differentmagnitude.
 6. The method of claim 2, further comprising: determining avoltage drift associated with one or more distributions of thresholdvoltages based at least in part on applying the first read voltage,wherein applying the second read voltage is based at least in part onthe voltage drift, the first read voltage, or both.
 7. The method ofclaim 1, further comprising: identifying one or more distributions ofthreshold voltages, wherein applying the first read voltage is based atleast in part on identifying the one or more distributions of thresholdvoltages.
 8. The method of claim 7, further comprising: identifying amagnitude, a polarity, or both of the first read voltage based at leastin part on identifying the one or more distributions of thresholdvoltages, applying the plurality of voltages as part of the pre-readportion, or both.
 9. The method of claim 1, further comprising:determining a voltage drift associated with one or more distributions ofthreshold voltages; and determining a shape of one or more distributionsof threshold voltages based at least in part on determining the voltagedrift, wherein applying the first read voltage is based at least in parton determining the shape, determining the voltage drift, or both. 10.The method of claim 1, further comprising: determining the first readvoltage based at least in part on applying the plurality of voltages aspart of the pre-read portion, wherein applying the first read voltage isbased at least in part on determining the first read voltage.
 11. Anapparatus, comprising: a memory array; and a controller coupled with thememory array and configured to cause the apparatus to: apply a pluralityof voltages as part of a pre-read portion of a read operation; apply afirst read voltage to a memory cell as part of a read portion of theread operation based at least in part on applying the plurality ofvoltages; identify a second read voltage that comprises a differentpolarity than the first read voltage based at least in part on applyingthe first read voltage; and identify a logic state stored by the memorycell based at least in part on applying the first read voltage and thesecond read voltage.
 12. The apparatus of claim 11, wherein to apply thesecond read voltage to the memory cell is done as part of the readportion of the read operation based at least in part on applying thefirst read voltage.
 13. The apparatus of claim 12, wherein thecontroller is further configured to cause the apparatus to: identifythat the second read voltage comprises a different magnitude as thefirst read voltage, wherein applying the second read voltage is based atleast in part on identifying the second read voltage.
 14. The apparatusof claim 12, wherein the controller is further configured to cause theapparatus to: identify a voltage drift associated with one or moredistributions of threshold voltages based at least in part on applyingthe first read voltage, wherein applying the second read voltage isbased at least in part on identifying the voltage drift.
 15. Theapparatus of claim 11, wherein the controller is further configured tocause the apparatus to: identify a magnitude, a polarity, or both of thefirst read voltage based at least in part on applying the plurality ofvoltages as part of the pre-read portion.
 16. The apparatus of claim 11,wherein the controller is further configured to cause the apparatus to:determine a voltage drift associated with one or more distributions ofthreshold voltages, wherein applying the first read voltage is based atleast in part on determining the voltage drift.
 17. A method,comprising: applying a plurality of voltages as part of a first portionof a read operation; applying a first voltage to a memory cell as partof a second portion of the read operation based at least in part onapplying the plurality of voltages; applying a second voltage to thememory cell as part of the second portion of the read operation based atleast in part on applying the first voltage, wherein the second voltagecomprises a different polarity than the first voltage; and determining alogic state stored by the memory cell based at least in part on applyingthe second voltage.
 18. The method of claim 17, further comprising:identifying a positive polarity of the second voltage based at least inpart on applying the second voltage, wherein determining the logic stateis based at least in part on identifying the positive polarity.
 19. Themethod of claim 17, further comprising: identifying a negative polarityof the second voltage based at least in part on applying the secondvoltage, wherein determining the logic state is based at least in parton identifying the negative polarity.